Electrophysiological Assessment of Auditory System: Middle Latency Response

Middle Latency Response (MLR)

• First reported by Geisler, Frishkoph, and Rosenblith in 1958
• MLR is a series of waveforms occurring from 10-80 ms after the onset of an auditory stimulus

Components of MLR

• Consists of 2 positive and 2 negative peaks, labelled as Na, Pa, Nb, and Pb
• Normal latency range for each peak:
  • Na: 16.25-30.00 ms
  • Pa: 30.00-45.00 ms
  • Nb: 46.25-56.25 ms
  • Pb: 56.25-80 ms
• Wave amplitudes range from 0.25 to 3.0 µV

Generators of MLR

• Multiple generators contributing to the waveform
• Secondary auditory cortex (Wernicke's zone-Brodmann's area 22) receives afferents from the dorsal and medial subnuclei of the medial geniculate nucleus
• Primary auditory cortex lies between the sylvian fissure and superior temporal gyrus
• Auditory cortex (primary/secondary) with possible upper brainstem and thalamic contributions

Peak Generators

• Na: Mostly from sub-cortical regions of the auditory system, specifically MGB of thalamus, thalamocortical region, and portions of the inferior colliculus and Heschl's gyrus
• Pa: Superior temporal gyrus within the auditory cortex
• Nb: Still unclear
• Pb: Auditory cortex, perhaps the posterior region of the planum temporale and even hippocampus

Nomenclature

• Goldstein & Rodman introduced the labels Na, Pa, and Nb in 1967 to describe the MLR components
• Other terminologies include Auditory Middle Response (AMR), Middle Latency Response (MLR), and Middle Latency Auditory Evoked Potentials (MLAEP)

Test Protocols

• Parameters:
  • Stimulus: Click or tone-burst signals
  • Transducers: ER-3A or insert earphones
  • Duration: 0.1 ms for click signals, 2 cycles for tone-burst signals